This invention relates to recording microelectrodes used for biological readings. In particular, it relates to a circuit and a method for compensating the floating differential capacitance appearing between the microelectrodes.
Faithful recording of biological signals with a single glass microelectrode cannot usually be done without compensating the undesired capacitances at the microelectrode boundaries with a negative capacitance created across the microelectrode by a negative impedance converter. Among the main undesired capacitances are the intrinsic input amplifier capacitance and a capacitance proportional to the length of the leads connecting the microelectrode to the amplifier and guards, used for shielding the microelectrode from external noise. Microelectrodes are used to measure electrical potentials on organisms such as humans, animals or even plants. Microelectrodes are able to pick up the small voltage fluctuations that arise from muscle, brain or neural activity. It is well known in the art how to apply such microelectrodes to an organism.
Not always negligible is a capacitance located at the tip of the microelectrode, which adds another small (or the order of a few pF) capacitance for each xe2x80x9cmmxe2x80x9d length of tip immersed in the preparation. Several methods have been developed for compensating these capacitance effects in order to achieve a desirable response of the recording system. A sophisticated technique for avoiding capacitive negative compensation is a follower with a buffer. It succeeds in reducing a large part of input capacitance by bootstrapping the amplifier head stage, but it does not always achieve full compensation because of the distributed nature of the microelectrode""s electrical parameters. Recording with two microelectrodes is more complicated because a differential floating capacitance appears between them, which cannot be compensated with existing grounded microelectrode capacitance neutralization circuits. There is then a need for compensating the floating differential capacitance.
Recording with two microelectrodes is desirable in many experimental procedures. When transient phenomena are not important, no special procedure is required and the experimenter can ignore any transient phenomenon and just look at steady state signals. Quite often though, experiments with two microelectrodes are designed for looking at short time effects, for example, injecting a current with one microelectrode and looking at the same time at the change reflected on the other recording microelectrode. A circuit (Axoprobe-1A from Axon Instruments Inc.) is already available commercially for compensating the effect of injecting a current in one microelectrode without seeing its effect on the recording microelectrode. However, this circuit is not symmetrical as it operates on one side only. One microelectrode is for recording and the other is for injecting current. Microelectrode roles cannot be switched.
FIG. 1, labeled as prior art, depicts a typical experimental set up when recording with two microelectrodes. It can be seen that two microelectrodes 15 and 14 (Rm1 and Rm2), each recording their own intra-cellular potential 17 and 18 (E1 and E2), are connected to their respective follower amplifier 10 and 11 (A1 and A2). As explained earlier, each microelectrode and amplifier contributes to the grounded capacitance 12 and 13 (Cm1 and Cm2) appearing across each microelectrode. These capacitances can be compensated provided a negative capacitance is generated as input impedance of the associated compensating circuit. However, due to the fact that the two microelectrodes are close together, another floating stray capacitance of differential nature 16 (Cd) appears between them. The value of this capacitance is dependent on the distance between the two microelectrodes, on the nature of the glass and most importantly on the length of the tips joined together at the end. It can be shown that this capacitance cannot be cancelled out with either amplifier compensating circuit. The cross-talk effect of this capacitance can corrupt the recording signals. In fact, any signal appearing on one microelectrode is also partially seen by the other through this differential capacitance. Since the quality of the readings is essential in many applications, compensating for this differential capacitance is important to ensure good interpretation of the results. Reproducing physiological signals with high fidelity is necessary for today""s medical world.
One aim of this invention is to provide a circuit and a method for compensating the differential floating capacitance between dual microelectrodes.
Another object of this invention is to permit a better reproduction of physiological signals when measuring with dual microelectrodes.
In accordance with the present invention, there is provided a method for processing signals from two microelectrodes to neutralize a floating differential capacitance that appears between the two electrodes. Readings of the potential at two points on an organism are taken. A differential amplifier is provided and connected to the two reading points. By adjusting an element of the differential amplifier, it is possible to cancel out the floating differential capacitance.
The differential amplifier can be composed of two amplifiers interconnected by an adjustable resistive element. When connecting the differential amplifier to the reading points, two capacitors can be introduced in the connection. Adjusting an element of the differential amplifier can consist in adjusting a resistive element in the differential amplifier. In order to cancel the floating differential capacitance, the output signals of the microelectrodes can by analyzed in different ways. For example, they can be displayed on an oscilloscope and an operator can adjust the element or the waveforms can be analyzed automatically and an adjustment can be provided.
In accordance with another broad aspect of the invention, a system is provided for synthesizing a negative floating capacitance matching a floating differential capacitance appearing between two microelectrodes. This system comprises two amplifiers connected to the two microelectrodes. It also comprises an adjustment circuit element interconnecting the negative inputs of these two amplifiers and the outputs of these amplifiers are fed-back to the microelectrodes. The outputs of the microelectrodes can be provided with followers amplifiers. The adjustment circuit element can be a variable resistive element and the amplifiers can be connected to capacitive elements.